The hypothesis that modern birds are the descendants of a group
of small dinosaurs, called dromaeosaurs,
part of a bipedal group called theropods, has become increasingly accepted by
the scientific community, to the point where it is very close to being a scientific
consensus (there is a tiny and shrinking minority of scientists who disagree).
Several important
finds, many from China, have been made in the last few years (1) - (9) which support this hypothesis. Fossils have been found
which demonstrate the evolution of feathers and the anatomical structures necessary
for flight.

The evidence
has persuaded almost the entire scientific community that birds evolved from
theropod dinosaurs. There is, however, one aspect of bird physiology that has
been a puzzle since the theory of dinosaur origins of birds was proposed. The
fact is that birds have a respiratory (breathing) system unlike that of almost
all other tetrapods (tetrapods include all mammals, reptiles, amphibians and
birds). The puzzle centres on the fact that birds seem to have a breathing system
different from all other living tetrapods without any antecedent - this is a
situation that creationists have, unsurprisingly, attempted to exploit. Now,
however, there is strong evidence that theropod dinosaurs that predated the
emergence of birds in the record had pulmonary systems like modern bird breathing
systems. This means that the flow-through lung is not unique to birds, but was
present in theropod dinosaurs before the evolution and emergence of birds.

The avian flow-through lung

All tetrapods (mammals, reptiles, amphibians, crocodilians)
apart from birds have a pair of lungs that operate on the bellows principle.
That is, a pair of lungs that inhale and exhale air by volume changes associated
with respiratory movements of the diaphragm and rib cage. The volume of the lungs
are increased by inspiratory effort drawing oxygen-rich air into the lung;
and are decreased to expel air partially depleted of oxygen and carrying waste carbon
dioxide, from the lungs.

Birds, however, have an entirely different respiratory
system. Birds breathe using a respiratory system that consists of a pair of
lungs and a number of separate air sacs that take up some considerable
space in the body cavity of the bird. Most birds have nine air sacs. The air
sacs are connected to the lungs in such a way that when a bird breathes air
is first drawn into the posterior air sacs. It then flows though the lungs
in a single direction into the anterior air sacs through a fine set of 'air
capillaries' (the parabronchi) in the lung. The air capillaries are closely
surrounded by blood capillaries and this is where the exchange of oxygen and
carbon dioxide takes place. The air sacs and lungs are arranged in such
a way that air flows in the same direction through the lungs whether the bird
is inhaling or exhaling (uni-directional flow-through). Unlike mammals and other tetrapods, the avian lungs
remain the same volume during breathing - the air is pumped by changes in the
volume of the air sacs not the lungs. The bird skeleton is highly pneumatized - that means
that there are large air spaces in the bones and vertebrae of birds which connect
with the air sacs.

Go to reference
10 for a detailed explanation of the avian respiratory system.

Recent finding

A recent paper
in Nature (11), shows that theropod dinosaurs have vertebrae pneumatized in
a way that is very similar to modern birds. The authors have investigated the
well preserved fossil of a theropod dinosaur called Majungatholus atopus and
have found that the vertebrae possess very close similaritiies in pneumaticity
compared with an extant bird (the sarus crane). See fig 1 below.

However, the
similarity between the pneumatic features of theropod dinosaurs and modern birds
was already known through a number of studies (12). So what does this
new study indicate that we didn't know before? Detailed analysis of the
individual vertebrae and ribs reveal a pattern of pneumaticity that is entirely
consistent with the pattern in living birds - that is, the cervical air sac
connect to vertebrae and ribs in the neck region of the spine, and in the thoracic
vertebrae nearest the head; the abdominal air sac connects with the tail and
sacrum vertebrae and the thoracic vertebrae nearest the tail; and the lung
itself connects with the mid-thoracic vertebrae. This pattern is the same in
all birds and is exactly what is found by detailed analysis of the vertebrae
of M atopus. So it is not the discovery of pneumatised vertebrae in this fossil
that is new, but the fact the pattern of pneumatisation is found to be the same
as in living birds and consistent with a uni-directional flow-through breathing system.
This situation is consistent for all known non-avian theropods, suggesting that
it is a derived characteristic of the first theropods and spread throughout
the entire clad of theropods including modern birds

Furthermore,
O'Connor and Claessens point out that in order for either uni-directional or
bi-directional flow-through ventilation to work, the tail end of the abdominal
cavity has to change volume more than the head of the cavity. Indeed that is
the arrangement in birds, and analysis of the skeleton of theropods shows that
they possess the appropriate characteristics in the articulation of the ribs
with the vertebrae to show that the tail end of the trunk can change volume
mure than the head end, just as in birds. Indeed air sacs and associated
features at the tail end of the abdominal cavity are known to have developed
in chameleons, snakes and certain types of lizard and this indicates that the
tail end of the lung in the entire sauropsid (a group that includes birds and
most reptiles and dinosaurs) is able to develop air sacs and invade the tail
end of the skeleton.

So, this recent
study has shown that non-avian theropod dinosaurs had the necessary anatomy
for flow-through ventilation similar to extant birds and, that in the evolution
of the flow-through system, the tail end air sacs likely developed before those
at the head end of the trunk.

Creationist
response

Carl Wieland
of 'Answers in Genesis' has written a response to this paper (13), that is generally
reasonably restrained, but utterly fallacious. He correctly points out
that this analysis does not, in itself, resolve the issue of the steps
by which a bellows type lung evolved into the avian flow-through system. His
discussion is, however fundamentally flawed in one important respect:
his main objection to the evolution of an avian system from a bellows system
is that he cannot see how it could happen. This of course is the old canard
(a term that is peculiarly well suited to this subject!) of the argument from
personal incredulity. Carl cannot conceive of a pathway by which the avian
lung could evolve from a bellows arrangement, so of course, in his mind, it
cannot have happened. This was the original design argument used by William
Paley. It was intellectual gruel then, and it is intellectual gruel now.
Carl would have us believe that there is an 'in-principle' barrier to
the evolution of 'flow-through' ventilation. (I think Carl means unidirectional
flow-through ventilation - he doesn't seem to recognise the distinction between
unidirectional and bidirectional flow-through ventilation). Of course, there
is no such 'in-principle' barrier and one can think of very obvious
routes by which the avian system could develop from a bellows arrangement. The
first obvious step in this process, the development of a bidirectional
flow-through system with air sacs positioned beyond the lung in the tail end
of the trunk is strongly supported by this study. Carl asks 'How could any creature breathe while the in-between stages were evolving, while air was
not yet flowing through but no longer going in and out?
The answer to this is obvious: it breathes by air flowing thriough the lung
bidirectionally - in other words it flows through and goes in and out.

I wonder whether
Carl realises that, even in modern birds, there is a mixture of unidirectional
flow through the so-called palaeopulmonic bronchi and bidirectional flow
through the so-called neopulmonic bronchi.

Carl also claims
to know clearly what constitutes the same and what constitutes different biblical
'kinds'. If so, he is the first man on earth to do so, as no creationist has
ever defined a 'kind' in a self-consistent way.

The fact is
that not only is the evidence very strong that birds evolved from theropod dinosaurs,
but there is no objection 'in principle' to development of the avian respiratory
system.